1. The Field of the Invention
The present invention relates to methods and apparatus for purifying and storing gases. More particularly, the present invention is related to methods and apparatus for purifying and storing hydrogen gas.
2. The Relevant Art
Hydrogen is one of the most ubiquitous elements in the universe as well as the most reactive, forming more compounds than any element in the periodic table including carbon. Hydrogen gas of high purity, having contaminants at part-per-billion concentrations, is used commonly in semiconductor fabrication and other "high-tech" applications.
High purity hydrogen gas is produced on an industrial scale by one of several techniques. The most common method of manufacture is the catalytic steam-hydrocarbon reforming process in which hydrocarbons from natural gas or refinery feedstock are reacted catalytically with water at elevated temperatures to form carbon monoxide (CO) and hydrogen gas (H.sub.2), as illustrated below where propane (C.sub.3 H.sub.8) is the feedstock hydrocarbon: EQU C.sub.3 H.sub.8 +3H.sub.2 O.fwdarw.3CO+7H.sub.2.
The carbon monoxide can be further converted to carbon dioxide (CO.sub.2) and hydrogen.
An alternative method involves the production of hydrogen as a by-product of the production of chlorine and sodium hydroxide (NaOH) by the well known brine electrolytic process in which the chloride ion present in aqueous sodium chloride is oxidized to form chlorine gas (Cl.sub.2) in addition to H.sub.2 and NaOH. In another method, useful for small scale generation of hydrogen, an equimolar mixture of methanol (CH.sub.3 OH) and water (H.sub.2 O) is vaporized and passed over a "base-metal chromite" type catalyst at 400.degree. C. wherein the mixture is cracked into hydrogen and carbon monoxide. The carbon monoxide subsequently reacts with the steam to produce carbon dioxide and more hydrogen.
Regardless of the method of production, the hydrogen gas typically must be purified to remove gaseous impurities such as nitrogen (N.sub.2). Nitrogen is especially prevalent in hydrogen produced by electrochemical methods. Commonly, a technique known as cryogenic absorption is used to remove nitrogen residues from hydrogen gas by precipitating the nitrogen and other gas impurities from the impure hydrogen gas mixture. In the cryogenic absorption process, the contaminated hydrogen gas is passed over charcoal immersed in a bath of liquid nitrogen. Although this method provides hydrogen gas having impurities on the order of parts-per-billion (ppb), the method is very complex and expensive to operate.
Other methods have focused on applying "getters" to purify hydrogen gas. As used herein, the terms "getter" and "scavenger" refer to substances that bind gas molecules by a "sorption" process, i.e., by adsorption or absorption. Unfortunately, using getters to sorb nitrogen and other impurities is impractical for nitrogen concentrations greater than about 20 parts-per-million (ppm) to 50 ppm due to depletion of the getter materials. In particular, oxygen (O.sub.2) N.sub.2, and H.sub.2 O contaminants commonly found in hydrogen feedstocks quickly contaminate the getter materials and degrade their performance.
Attempts to store hydrogen in getters or other hydride forming materials and "flush" the non-sorbed impurities have also proved inefficient as such systems must be capable of absorbing large amounts of energy released during the sorption process and yet retain the capability to produce the elevated temperatures required to release the hydrogen from the getters after the impurities have been expelled from the system. Such systems are therefore highly inefficient in terms of energy use.
Additional factors complicating this process include difficulties in ensuring that the "void volume" remaining after sorption of the hydrogen is sufficiently free of residual impurities, and the loss of hydrogen gas due to leaks in the system. Purging the void volume is critical to producing purified gas, as the degree of gas purity is limited by the amount of impurities remaining in the void volume when the purified gas is released from the getter material for use. Thus, the void volume must be rigorously purged with gas that is at least as pure as the gas of desired purity. The purge gas must then either be itself purified again, or released from the system. Thus, it will be seen that present getter purification methodologies waste purified gas. For example, as much as 10-20% of purified hydrogen may be lost through purging to achieve gas purities on the order of 10-50 ppb. Still more purified hydrogen gas would be lost in achieving purification levels on the order of 1 ppb. The result of this limitation is that present methods of purification employing getters can provide only limited purification economically, and are used to reduce impurity concentrations to between about 500 ppb-100 ppb. These methods are not capable of providing hydrogen gas having impurity concentrations of less than about 10 ppb economically. Furthermore, getter purification technologies are also not economical for hydrogen gas having large concentrations of nitrogen, as exposure to nitrogen exhausts getter capacity quickly. These limitations with existing getter methodologies are especially difficult for third world countries that are attempting to develop modern electronics industries that require sources of highly purified hydrogen gas but cannot afford the expense associated with cryogenic methods of hydrogen purification.
Still another method for purifying hydrogen gas uses a palladium or palladium alloy diffuser separator to separate hydrogen preferentially from mixtures of gases, such as those described in U.S. Pat. Nos. 3,368,329 and 3,534,531 to Eguchi, et al., each of which is incorporated herein by reference. Generally, impure hydrogen gas is passed through a palladium or palladium alloy membrane (ie., a continuous walled metal matrix structure that functions as a diffusion barrier to selectively allow the diffusion of hydrogen through the metal matrix), or packed small-diameter, thin-walled tubes of palladium or palladium alloy, to purify the hydrogen gas. Unfortunately, such methods require the use of high pressures (about 6 Bar) to provide flow rates which are economical for use in semiconductor fabrication processes or other processes where a steady flow of hydrogen gas is required. Furthermore, the need to maintain a large pressure differential across the metal purifier requires the use of pumps which introduce additional sources of leakage and contamination. Because of the high pressure requirement, palladium and palladium alloy methods of purification are practical only for point-of-use purification, e.g., in the laboratory, or for applications where high flow rate is not a requirement.
Thus, there remains a need for economical and reliable methods and apparatus for purifying hydrogen gas on a large scale and at a flow rate which is adequate for modern industrial requirements.